Portable life support systems are used in a variety of situations in which the ambient environment around the user cannot be breathed either because of the lack of oxygen in usable form or because of the presence of substances which would have toxic effects if inhaled. These uses include extravehicular activity in space, scuba diving, deep off-shore diving work, use in contaminated atmospheres, use at high altitudes and the like.
The two fundamental architectures in the design of portable life support apparatus are open circuit and closed circuit systems. Open circuit systems, typified by the underwater diving system popularized by Jacques Cousteau, are the simplest, consisting of a compressed gas supply and a demand regulator, or mouthpiece, from which the user breathes. The exhaust gas is ported overboard with each breath, hence the name "open" circuit. These systems are bulky and inefficient in that the oxygen not absorbed during each breath is expelled and wasted. Additionally failure of any component results in failure of the system.
Closed circuit systems, also known as rebreathers, make nearly total use of the oxygen content of the supply gas by removing the carbon dioxide generated by the user, and adding makeup oxygen or oxygen containing gas to the system when the internal volume drops below a set minimum level, or when the oxygen partial pressure drops below some pre-established setpoint.
These closed circuit breathing systems generally consist of a mouthpiece from which the user breathes and which is connected by means of two flexible waterproof hoses, one to remove the exhaled gas and the other to return the processed gas, to a means for removing the carbon dioxide from the breathing gas, replenishing metabolized oxygen, and providing for makeup gas volume with a breathable gas to maintain system volume during descent as the gases within the breathing circuit are compressed. Such devices are usually provided with a series of checkvalves located near the mouthpiece such that gas flow within the breathing circuit is always maintained in a single direction. Oxygen addition to the system may be made by oxygen generators, such as the type disclosed in U.S. Pat. No. 2,710,003, to Hamilton et al., or the addition of oxygen or an oxygen containing gas either through a constant mass flow orifice or by means of a manually operated or a sensor-controlled electronic valve.
The mouthpieces from either an open-circuit or a closed-circuit mode may be combined with other breathing devices or with each other. U.S. Pat. No. 4,304,229 illustrates a mouthpiece which may be switched in a snorkel mode and an open-circuit mode.
The prior art mouthpieces capable of being switched between open and closed circuit modes, such as shown in U.S. Pat. No. 4,273,120, have been complex, bulky devices with high breathing resistance that are difficult to operate.